ARS | Publication request: Open field host selection and behavior by tamarisk beetles (Diorhabda spp.)(Coleoptera: Chrysomelidae) in biological control of exotic saltcedars (Tamarix spp.) and risks to non-target athel (T. aphylla) and native Frankenia

Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 16, 2009
Publication Date: N/A

Interpretive Summary: Saltcedar was introduced over 100 years ago from Asia and the Mediterranean for erosion control, but this small tree has since formed dense thickets over millions of acres of land in the western U.S., reducing agricultural and recreational use. This plant also removes water and adds salt to the soil as it grows, and displaces the plants that normally grow alongside waterways and the animals that depend on them. In the late 1990s, scientists with the USDA-ARS discovered, tested and released the saltcedar leaf beetle as part of an effort to biologically control this weed. The beetles have already devoured thousands of acres of saltcedar. In this study, we evaluated the ability of the saltcedar beetle to do damage and establish itself in the field on athel, a beneficial exotic tree that is closely related to saltcedar. Athel is used in the southwestern U.S. and in Mexico as a shade and windbreak tree. We began by confining beetles on branches of saltcedar or athel, and found that female beetles laid two to four times as many eggs on saltcedar as on athel, and that larvae destroyed both saltcedar and athel foliage inside bagged branches. In 2005 and 2006, we released hundreds of beetles on saltcedar trees growing alongside planted athel trees at a site in South Texas. We also planted saltcedar and athel in test plots positioned near mature saltcedar trees that were being destroyed by thousands of beetles in west-central Texas and in Nevada. At all three sites, beetles flew to both saltcedar and athel trees, but laid two to five times more eggs on saltcedar than on athel, and failed to establish permanent populations on athel. These results suggested that the negative impacts of saltcedar beetles on athel are minimal, supporting the use of saltcedar beetles as a control tool in areas in which athel trees grow.

Technical Abstract: Biological control of exotic, invasive saltcedars (Tamarix spp.) in the western USA involves releases of exotic saltcedar leaf beetles, Diorhabda elongata Brullé sensu lato. Adults in cages alight, feed and oviposit on athel (Tamarix aphylla), an evergreen cold-intolerant tree used for shade and as a windbreak. The ability of D. elongata beetles to alight, oviposit, and produce larvae on athel was investigated in the field. In no-choice field tests, saltcedar beetles from Crete, Greece and Sfax, Tunisia produced 30¿45% as many egg masses and 40¿60% as many larvae on athel as on saltcedar. Adult, egg mass and larval densities were at least 10-fold higher on saltcedar than on adjacent athel trees after two weeks at a field site in south Texas, and damage caused by the beetles was 2¿10-fold greater on saltcedar over ten weeks. The beetles did not establish populations on saltcedar or athel at this site. At a site in west-central Texas, egg masses and 1st and 2nd instars were 3¿9-fold more abundant on saltcedar than athel saplings in pots near a mature saltcedar stand being defoliated by an established Crete beetle population. At a site in northwestern Nevada, 3rd instars of a Fukang, China population were 2.5-fold more abundant on saltcedar than on athel saplings in plots planted near mature, defoliated saltcedar trees. The results suggest that selective oviposition by adult D. elongata reduced egg and larval populations by 50¿90% on athel relative to saltcedar in the field. The impacts of D. elongata on athel coincident to releases onto saltcedar are thus likely to be limited.